linux-kernel - Re: [PATCH v12 2/5] perf: Add a counter for number of user access events in context

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Date:   Fri, 3 Dec 2021 09:58:15 -0600
From:   Rob Herring <robh@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v12 2/5] perf: Add a counter for number of user access
 events in context

On Tue, Nov 30, 2021 at 4:57 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Wed, Oct 27 2021 at 15:16, Rob Herring wrote:
> > For controlling user space counter access, we need to know if any event
> > in a context (currently scheduled or not) is using user space counters.
>
> Who is 'we'?
>
> Come on. How is someone without context supposed to figure out that 'we'
> means ARM64 when staring at that changelog a year later?
>
> Is it really that hard to write coherent changelogs which make sense on
> their own and actually follow Documentation/process/ which exists for a
> reason?

I've rewritten it like this:

On arm64, user space counter access will be controlled differently
compared to x86. On x86, access in the strictest mode is enabled for all
tasks in an MM when any event is mmap'ed. For arm64, access is
explicitly requested for an event and only enabled when the event's
context is active. This avoids hooks into the arch context switch code
and gives better control of when access is enabled.

In order to configure user space access when the PMU is enabled, it is
necessary to know if any event (currently active or not) in the current
context has user space accessed enabled. Add a counter similar to other
counters in the context to avoid walking the event list every time.